Reseach has focused on determining the genetic and physiological basis of malaria refractoriness manifested by a selected strain of Anopheles gambiae. This mosquito strain exhibits the capacity to block sporogonic development of a wide range of Plasmodium species (includiing Plasmodium berghei, Plasmodium knowlesi, several isolates of Plasmodium cynomolgi, and certain isolates of Plasmodium falciparum). The block is manifested in all cases by the appearance of encapsulated oocysts on the midgut of infected mosquitoes. Preliminary data suggest that inheritance of refractoriness is multifactorial, with one locus responsible for most of the difference between refractory and susceptible lines. The allele for refractoriness at this locus is recessive. The expression of refractoriness is also highly correlated with a particular esterase isozyme. The refractory line is monomorphic for this esterase type and individual mosquitoes from the unselected, parent strain of A. gambiae which exhibit this isozyme have invariably been found refractory.